1. Field of the Invention
Sodium-sulfur batteries are looked to as one possible solution to the storage of electrical energy for vehicular and electric utility application. The operating principles as well as the unique advantages of the sodium-sulfur-solid electrolyte battery have been discussed by Marcoux and Seo, "Sodium-Sulfur Batteries," Advan. Chem. Ser., No. 140, 216 (1975). The sodium-sulfur battery has a number of unique characteristics which distinguish the sodium-sulfur battery from other secondary batteries which are normally encountered. In the sodium-sulfur battery, the electrodes are liquids. The temperatures at which the sodium-sulfur battery is employed are above the melting points of sodium, sulfur and sodium polysulfide. The electrolyte is a solid, commonly .beta.-alumina, which has mobile sodium cations. In addition, sulfur which forms the cathode is a non-conductor and, therefore, means must be provided for the transfer of electrons to and from the sulfur on charging and discharging. Finally, the initial product of the battery discharge is sodium pentasulfide (Na.sub.2 S.sub.5) which is immiscible with sulfur, so as to create an inhomogeneous mixture, which is comprised of the ionically conducting sodium pentasulfide and the non-conducting sulfur. The sulfur can, therefore, interfere with the sodium transport and electron transfer. Upon further discharge reaction, the two-phase mixture of sodium pentasulfide and sulfur disappears to form a single phase with an average composition between sodium trisulfide (Na.sub.2 S.sub.3) and sodium pentasulfide, depending upon the state of charge. When the battery is substantially charged, the two phase region is again formed. However, it has been found, in sodium-sulfur batteries employing a .beta.-alumina electrolyte separator and porous carbon, as the electronic conductor for the sulfur, that upon repeated charging and discharging the capacity of the battery diminishes rapidly to a relatively low percentage of the initial capacity of the battery. Once the two phase region is reached during charging, further charging is only done with considerable difficulty. It has been concluded that the sulfur, being non-conductive, forms a blocking or insulating layer which prevents further reaction. This result is extremely undesirable, since it means that the theoretical capacity of the battery is lost and the system only partially employs the electrochemical energy available from the sodium and sulfur which is employed. In addition, partial blocking of the electrolyte causes localized high current densities at certain points within the cell. This, in turn, can cause cell failure and, therefore, limited life capability.
2. Brief Description of the Prior Art
In a National Science Foundation report, entitled "Research on Electrodes and Electrolyte for the Ford Sodium-Sulfur Battery," Steven A. Weiner, July 1975 (Contract No. NSF-C805) AER-73-07199, the problem of efficient recharing is discussed and a solution described based on the notion of enhanced convection of the reactants present in the sulfur electrode.